Photochromism is a phenomenon which has drawn attention in the last several years, and is a reversible reaction in which a certain compound quickly changes its color when irradiated with an ultraviolet-containing light such as sunlight or a light emitted from a mercury lamp and, when the light irradiation is stopped and the compound is placed in a dark room, returns the original color. Compounds capable of photochromism are called photochromic compounds, and photochromic compounds having various structures have heretofore been synthesized. These conventional photochromic compounds have poor durability.
There are known novel photochromic compounds having improved durability, such as spirooxazine-based photochromic compounds (hereinafter referred to simply as oxazine compound), fulgimide-based photochromic compounds (hereinafter referred to simply as fulgimide compound), chromene-based photochromic compounds (hereinafter referred to simply as chromene compound) and the like. See U.S. Pat. Nos. 4,882,438, 4,960,678, 5,130,058 and 5,106,998; and Japanese Kokai (Laid-Open) Patent Application Nos. 62-288830, 2-28154, 3-11074 and 3-133988!.
Also in Japanese Kokai (Laid-Open) Patent Application No. 5-306392 is proposed combination use of a compound having an epoxy group(s) as a component for further improving the durability of fulgide compound.
Japanese Kokai (Laid-Open) Patent Application No. 187784/1987 discloses a process for producing a photochromic resin, in which a highly reactive polyfunctional (meth)acrylate monomer is used and the concentration of polymerization initiator is specified, in order to prevent the degradation of an oxazine compound caused by polymerization initiator when the oxazine compound is mixed with and dissolved in a radically polymerizable monomer (hereinafter referred to simply as monomer) and curing is conducted in the presence of a polymerization initiator.
The above-mentioned photochromic compounds show excellent reversible durability. Among them, the oxazine compound is known as compound which generally hardly undergoes photodegradation, and when subjected to continuous irradiation with sunlight or a light similar thereto, shows a small reduction in initial color-developing performance and hence, exhibits excellent performance in light resistance. However, in order to easily produce a cured product by a method comprising mixing and dissolving the above photochromic compound and a monomer and subsequently subjecting to polymerization for curing (hereinafter referred to simply as a knead-mixing method) and use the cured product in various applications, it is necessary to develop a process for producing a cured product, in which the characteristic properties of the above oxazine compound can be exhibited fully.
For example, a cured product obtained by knead-mixing diethylene glycol bisallylcarbonate with the above-mentioned photochromic compound, which is widely used as a spectacle lens, has a problem in that the photochromic compound is degraded by the action of radical seed in curing and consequently, the cured product has a reduced initial color-developing performance.
In the process disclosed in Japanese Kokai (Laid-Open) Patent Application No. 62-187784 disclosing a method comprising curing a polyfunctional (meth)acrylate monomer and an oxazine compound at a polymerization initiator concentration specified in the literature by the knead-mixing method, a cured product capable of exhibiting sufficient photochromic performance can be obtained when a fully purified polyfunctional (meth)acrylate monomer is used. When a polyfunctional (meth)acrylate monomer generally available in the market and an oxazine compound are mixed, however, the resulting solution is strikingly colored, and when the solution is cured according to the process described in the literature, the resulting cured product not only is strikingly colored but also exhibits extraordinarily reduced color-developing performance.
As described above, a photochromic cured product produced according to the knead-mixing method, using an oxazine compound and an insufficiently purified commercial polyfunctional (meth)acrylate monomer has not exhibited satisfactory performance.